The seven treasures quest is very tricky because the lists we have are not consistent, mostly because the earliest Buddhist sutras mentioning it are in Sanskrit and Chinese translators interpreted the materials differently over the centuries. Also, the earliest mentions are in terms of spiritual treasures and the identification with actual gems came later.
So you have all sorts of stuff which adds to the mystery -there's beryl, quartz, nacre, all sort of corals, carnelian, amber, silver, even jade.
Now our master scholar adds to it because his legs and arms are *technically* made up of two different materials each, so that's another problem.
I think the simplest solution is to consider every new 'piece' of Phos as one material because the spiritual ascension process makes it easier to identify single moments as a step towards enlightenment through a lesson learned. I admit that it leaves the problem of silver being in every listing of treasures open when the easiest thing would be to consider the gold-platinum alloy as representing gold and silver altogether, but who knows if Phos isn't going to get some silver too.